I am sure the Prince was very well briefed but he seemed surprised that so many members of his family had signed the Bible. He seemed genuinely touched to see so many of their signatures there.

Rev Justin Moffatt, Rector of St Phillip's Church in Sydney

They added their names to the First Fleet Bible, which was used in the first Christian service held in Australia in 1788 and the Book of Common Prayer, signed by generations of royals.

The Queen signed it in 1954. William’s parents, the then Prince and Princess of Wales, added their names in 1983, the Duke and Duchess of York in 1988 and Queen Elizabeth the Queen Mother in 1958.

As they chatted with Rev Justin Moffatt, Rector of St Phillip's Church in Sydney, where the priceless artefacts are kept under lock and key, the Duchess complemented members of the clergy on the service.

"It was absolutely beautiful," she said.

Crowds turned out to greet Wills and Kate as they left the church after an Easter service [PA]

William joked that the Bible was "in good nick" and appeared genuinely touched to see so many of his family - most notably his late mother, Princess Diana - had signed the book.

William and Kate wrote their names first in the Bible - below that of Charles and Diana - and then in the Book of Common Prayer.

Afterwards, the Rev Moffatt with his wife Laurel, who showed the royal visitors the books, said: "I am sure the Prince was very well briefed but he seemed surprised that so many members of his family had signed the Bible. He seemed genuinely touched to see so many of their signatures there."

He added: "Prince William also joked to me 'It's in very good nick!' And I told him that it was very well looked after indeed. It is kept safe in our museum.

The Duke of Cambridge speaks with members of the church [PA]

"He also seemed very interested when we showed him the prayer for King George III and the Royal Family and was rather touched when I turned to the page of the Solemnization (c) of Matrimony which was covered with ink blots from the signatures of the first Christians to marry in Australia.

"I explained to him that the first convicts to settle here held those pens. The Prince seemed fascinated."

Afterwards the couple chatted to members of the choir and the Cathedral's Sunday School, whose pupils had made her a card.

Kate honed in on the youngest chorister, Freddie Cobb, six, and spoke to his father, Ross, who is director of music at the cathedral.

He said: "The Duchess said to Freddie: 'I had my eye on you right through the service. You were wonderful."